Porous structures are increasingly preferred in energy, biomedical, aerospace, chemical and other industries for various applications such as filtration and separation, distribution and control of fluid, electromagnetic shielding, heat exchanger, energy absorption, electrode matrices, and reaction matrices, due to their outstanding properties such as low specific weight, controlled permeability, large specific surface area, high energy absorption, and good absorption of electromagnetic waves. Specific examples of micro size porous parts include applications, such as wearable electronics and micro fuel cells.
Existing powder metallurgy and additive manufacturing (AM) based processes, such as micro metal injection molding and laser micro sintering, are capable of producing porous metal micro parts. However, these existing processes have several inherent drawbacks, such as the need for complex micro molds and the difficulty of filling feedstock completely into narrow cavities in micro injection molding. Further, laser and other thermal processes often have inevitable thermal effects such as thermal residual stress, cracking, and burr formation. More particularly, some AM processes such as Selective Laser Melting (SLM) and Electron Beam Melting (EBM) suffer from very high residual stresses due to the complete melting of the material during manufacturing. For example, the residual stresses caused by the thermal gradient associated with sintering processes are very high (>200 MPa). Another major roadblock that limits the application of the AM parts is that the part generally tends to have anisotropic properties due to the layered nature of the manufacturing.
Other complexities associated with traditional AM processes include the need for complex machining systems such as vacuum chambers, as in the case of Selective Micro Laser Sintering, to avoid humidity and resulting oxidation. One of the problems of inkjet process is that the viscous dissipation of binder fluid results in orifice clogging, which impedes ejection of the binder fluid through the nozzle. Also, the jetting process must be performed in a low oxygen environment to prevent the formation of a surface oxide layer, thus, resulting in changes to the physical properties of the jet surface. In summary, existing additive manufacturing techniques have several limitations ranging from a restricted choice of work material, to anisotropy, strength, scalability, internal stresses, and poor layer binding, resolution, and surface finish. Therefore, there is a clear need for alternate manufacturing approaches to produce porous micro parts without aforementioned constraints.